This invention relates generally to silicon mass air flow (MAF) sensors and in particular to an improved means for structurally strengthening the diaphragm in the region of the hot element.
While a silicon micromachined MAF sensor can offer certain benefits over one which comprises wound wire elements, the long-term survival of the former in "dusty" air flows has not been demonstrated to the degree that an automotive manufacturer whose products incorporate the latter for measuring the engine's induction air flow is ready to change over. In order to attain a possibility of acceptance by the automotive manufacturer, a silicon micromachined MAF sensor must survive accelerated dust testing under a condition where hundreds of grams of dust are flowed past the sensor within a few hours, and at rather substantial flow rates in terms of grams of dust per hour.
One improvement to the life expectancy of a silicon micromachined MAF sensor is attained by the use of a dust deflector which "shades" the sensor die from direct impacts. Examples of such technology are presented in commonly assigned Ser. No. 07/474,429 filed 02/02/90.
The improvement which is the subject of the present invention does not involve the use of a deflector element; rather, it involves a novel structure for the reliable long-term integrity of the silicon, consonant with the mandate that the sensor possess a fast and accurate response to the induction air flow, particularly to rapidly changing flow rates which can occur in the engine's air induction system.
The basic operating principle of a silicon micromachined air flow sensor is the same as that of a hot wire, or hot film, anemometer which dissipates power to the air flow in proportion to the flow velocity. (A reference sensor which does not dissipate power is used for temperature compensation.) In a silicon air flow sensor, a thin conductor layer (gold, for example) is the hot element. The response of the hot element to the air flow is optimized by minimizing the "heat sink" effect of the sensor structure which supports the hot element, i.e. minimizing heat transfer from the hot element to the support structure. In this way the power input to the hot element will be maximally transferred to the air flow.
One way of minimizing this "heat sink" effect in a silicon MAF sensor is by minimizing the thickness of the diaphragm in the region which supports the hot element. This region, which may be fabricated to only 1 micron thickness for example, is also designed to withstand a certain differential pressure, one atmosphere for example. The requirements imposed on the hot element support region of the silicon for the purpose of sensor responsiveness render the region prone to dust-impact-caused failure when subjected to testing like that described above.
The present invention relates to a support structure for improving the long-term survivability of the diaphragm in a dust-impacting environment without compromising the MAF sensor's accuracy and speed-of-response. Moreover, the invention attains this improvement without adding unacceptable cost or complexity to the fabrication process.
Briefly, the invention comprises a support structure consisting of a matrix of hollow glass microspheres distributed throughout an epoxy binder and disposed to provide backside support of the very thin region of the diaphragm whose frontside contains the hot element.
Further features, advantages, and benefits of the invention will be seen in the ensuing detailed description of a presently preferred embodiment constructed in accordance with the best mode contemplated at this time for carrying out the invention. Drawings accompany the disclosure and are briefly identified as follows.